This project seeks to improve our understanding of the detailed circuitry of the thalamus and it. relation to arousal, sleek- wakefulness cycles, and information transfer from subcortical relays to the cerebral cortex. Recent evidence suggests that the reticular nucleus of the thalamus reticular nucleus is critical for the generation of some rhythmic oscillation. which characterize forebrain activity during slow-wave sleek. The focus of this study is to identify the afferent inputs which might influence the excitability of neurons within the thalamic reticular nucleus and to determine the details of the relationship of these afferents to the cell bodies and dendrites of thalamic reticular nucleus neurons. During the past year, the external pallidal projection upon the squirrel monkey thalamic reticular nucleus was examined in the electron microscope following injections of WGA-HRP into the external segment of the globus pallidus and post-embedding immunohistochemistry for GABA using colloidal gold. The most common GABAergic synaptic profiles within the monkey thalamic reticular nucleus neuropil are large boutons packed with many pleomorphic vesicles as well as mitochondria. These boutons stain densely for GABA, are-always presynaptic, and establish symmetric synaptic contacts upon the somata and dendrites of thalamic reticular nucleus neurons. A small number of GABAergic profiles which participate in serial synapses was also found within the thalamic reticular nucleus neuropil. The GABA staining within profiles which participate in serial synapses is weak and they contain fewer vesicles and mitochondria than the F terminals. The external pallidal terminals, labeled with HRP, arise from thinly myelinated axons, and resemble F terminals; viz. they are large, packed with many pleomorphic vesicles and mitochondria, are always presynaptic, stain densely for GABA, and establish symmetric synapses frequently upon the somata and proximal dendrites of thalamic reticular nucleus neurons. Details of the relation of these to other extrinsic GABAergic inputs known to enter the thalamic reticular nucleus remain to be examined.